1. Field of the Invention
The present invention relates to an optical writing device, an image forming apparatus, and a method and program product for controlling the optical writing device. More particularly, the present invention relates to controlling an optical writing device capable of forming/outputting an image on the basis of image data in which each pixel is represented by multilevel data.
2. Description of the Related Art
In recent years, digitalization of information is a trend and this trend is likely to be further promoted. Therefore, there is indispensable an image forming apparatuses such as a printer and a facsimile machine for outputting the digitalized information, as well as an image forming apparatus such as scanner for digitalizing information from analog documents. Such an image forming apparatus is often provided with an image pickup function, an image forming function, a communication function and the like, so that the apparatus can be configured as a multifunction peripheral that is usable as a printer, a facsimile machine, a scanner, and a copier.
Among these image forming apparatuses, an electrophotographic image forming apparatus is widely used as the image forming apparatus for outputting the digitalized information. In such an electrophotographic image forming apparatus, a photosensitive element is optically exposed to form an electrostatic latent image. And, the electrostatic latent image is developed with a developer such as toner or the like to form a toner image. And, the toner image is transferred to a sheet such as paper to be outputted.
As for the electrophotographic image forming apparatus, there are known two types of optical writing devices, an LD (Laser Diode) raster optical system type and an LEDA (Light Emitting Diode Array) head type. In the LEDA head type, an LED array chip in which a plurality of light sources are aligned in line is used for exposing the photosensitive element for each main scanning line. As the LED light source, there are known two types of light sources, a binary controlled light source (hereinafter referred to as “binary head”) for controlling two statuses, “a switch on status” and “a switch off status”, and a multilevel controlled light source (hereinafter referred to as “multihead”) capable of realizing a gradation by changing the emission intensity.
However, in the multilevel controlled light source, the emission intensity may vary among the plurality of light sources because of the manufacturing tolerance or the like. And, the variation of the emission intensity may induce the variation of the exposure intensity on the photosensitive element, and thereby the variation of the image density. Therefore, under existing conditions, the binary head is often used as the light source of the optical writing device.
As a method of realizing a gradation expression in the optical writing device using the LED light source, there is proposed a method of dividing a main scanning line per pixel into a plurality of sub lines, and setting different exposure energies for each sub line, and selecting the sub line to be turned on in accordance with the gradation value of each pixel (for example, Japanese Patent Application Laid-open No. 2007-245537).
Even in a case that the binary head is used for the LED light source of the optical writing device, there is need to form and output an image including the gradation expression such as grayscale or the like. In a case that a controller of the image forming apparatus is configured in accordance with the optical writing device, a function for realizing the gradation expression with the binary head may be mounted on the controller side. Therefore, there is no need to arrange a special configuration on the optical writing device side.
On the other hand, in a case that the controller and the optical writing device of the image forming apparatus are designed independently of each other, the function for realizing the gradation expression with the binary head needs to be mounted on the optical writing device side, in order to provide the optical writing device applicable to various types of controllers. In a case that the method disclosed by Japanese Patent Application Laid-open No. 2007-245537 is employed for example, however, there is need to provide a plurality of line memories for each sub line, and to control the exposure intensity for each sub line. Thereby, the control system becomes complicated, resulting in increased circuit size or manufacturing cost.